Suppression of abnormal grain growth in the repair welding zone of cast magnesium rare earth alloys: Al₂RE phase pinning effect

This study addresses the issue of abnormal grain growth (AGG) occurring in the tungsten inert gas (TIG) repair welding of traditional Zr-refined Mg–4Y–3Nd–0.5Zr (WNZ4305) alloy during solution treatment. It proposes using an Al-refined Mg–4Y–3Nd–1.5Al (WNA4315) welding rod to replace the traditional Zr-refined welding rod, thereby improving the thermal stability of the repair welding microstructure. By comparing the solidification microstructure and second-phase composition of repair welding made with the two welding rods, investigating the microstructural evolution before and after solution treatment, the mechanism by which the WNA4315 welding rod suppresses AGG was systematically studied. Both Al-refined and Zr-refined rods significantly refine the grain size in the fusion zone (FZ) (to 13.4 ± 0.3 μm and 13.8 ± 0.4 μm, respectively), and the repair zone exhibits high residual stress. After solution treatment at 520 ℃for 8 h, serious AGG occurred in the repair welding zone of WNZ4305 electrode, and the average size of abnormal growth grains was 211.7 ± 2.8 μm. In contrast, the repair welding zone of the WNA4315 welding rod maintained fine equiaxed grains (16.4 ± 0.4 μm) without AGG. The FZ of the WNA4315 welding rod contains numerous thermally stable Al₂RE phases, which remain undissolved after solution treatment and effectively inhibit grain boundary migration through Zener pinning, significantly enhancing the grain thermal stability of the repair welding zone. This research provides a theoretical basis and process guidance for developing welding rods with high thermal stability for magnesium-rare earth alloys.